Process for control of food pathogens

ABSTRACT

A process for preserving canned, low-acid foods comprising the steps of adding diacetyl, which may be in the form of a starter distillate, to food and heating the canned food to render the food commercially sterile. The diacetyl lowers the temperature required for processing and remains in the food product at a level sufficient to kill germinating bacterial spores.

I. DESCRIPTION

1. Field of the Invention

This invention relates to methods for achieving botulinum safety inlow-acid canned foods by the use of diacetyl compounds in conjunctionwith heat.

2. Background of the Invention

In order to render canned foods safe and to prevent them from spoilingunder the usual conditions of storage, some form of commercialsterilization is required. The current methods of preservation of foodsinclude pasteurization and pressure sterilization.

Control of foodborne bacterial pathogens such as Salmonella, Shigella,enteropathogenic Escherichia coli, Campylobacter sp., and Yersinia sp.is currently achieved by heat treatments such as pasteurization (62.8°C. for 30 minutes or 71.7° C. for 15 seconds). Heat treatments higherthan pasteurization (100°-115° C. for varying periods but equal to F121Cof 3 or 0.3 minutes) are used to provide Clostridium botulinum safety inlow-acid canned foods and canned cured meats. Low-acid products have apH of 5.09 and above and include most meat and marine products, corn,peas, lima beans, asparagus and spinach. In the latter products,inhibition of surviving spores from the Clostridium botulinum is oftenachieved by NaNO₂ and/or potassium sorbate. The basis for the heattreatment required to render canned foods "commercially sterile" is thedestruction of Clostridium botulinum spores that produce neurotoxins.The processing failures (lower heat treatment than an F. value of 3minutes) or recontamination due to leaks in the can present seriousthreat to public health safety. There have been large recalls of cannedproducts that were associated with human deaths in the United States.

Although the high heat treatments tend to provide long-term storage offood at room temperature, they demand high levels of energy and are,therefore, expensive.

Searches for chemical compounds that are natural components of foodsthat may have antimicrobial properties indicated that -diacetyl, whichis a flavoring additive used in the food industry, may inhibit certaintypes of bacteria such as Escherichia coli, Salmonella andStaphylococcus. However, the effectiveness of diacetyl on inhibition ordestruction of bacteria varies by the bacteria type and also varies withthe growth medium. Testing previously has indicated that diacetyl isleast inhibitory or destructive in cooked meat or brain heart infusionmedia. As reported by author James M. Jay in "Antimicrobial Propertiesof Diacetyl," Applied and Environmental Microbiology, Vol. 4, No. 3,page 525 (Sept. 1982) diacetyl is ineffective against clostridia underanaerobic conditions.

BRIEF SUMMARY OF THE INVENTION

The process of the invention involves the use of specific levels ofdiacetyl, particularly in the form of a starter culture metabolitesolution combined with low level heating on the order of between 75 and110° C. to kill bacterial cells. During storage, this process will alsokill the germinating cells from spores that are normally associated withspoilage and/or botulism. In particular, it was found that the processof the invention provides effective inhibition of Clostridium inaddition to the foodborne bacterial pathogens described above which aremore readily controlled than Clostridium botulinum.

DETAILED DESCRIPTION OF THE INVENTION

A composition known as commercial starter distillate which containsdiacetyl is available from Chris Hansen's Laboratory, Inc. of Milwaukee,Wisconsin. The preparation of the starter distillate is well known andis described in the Federal Register of the United States at Vol. 48,No. 221, pages 51906-1983 and Vol. 47, No. 152, pages 34155-8 in 1982.Typically, the starter distillate is the distillation product of aculture of Streptococcus lactis subspecies diacetilactis which is grownin skim milk. The distillate includes diacetyl, lactic acid andethylacetate. The compound is typically utilized as a flavoring additivesince diacetyl has a butter-like flavor. About 2 percent of the starterdistillate is diacetyl. The starter distillate is available from ChrisHansen's Laboratories Inc. in a 15 ppm or 1,000 ppm solution. Typically,the starter distillate is used as a flavoring additive in quantities ofabout 0.01% to 0.5% in substances such as margarine or to flavorpopcorn. Members of the genus Leuconostoc are also used to producediacetyl.

S. aureus, S. typhimurium and E. coli were grown in brain heart infusionbroth (BHI) for 18 hours at 37° C. and a predetermined amount of eachwas added to BHI or whole milk to yield 10⁷ per mil initial levels ofbacteria. Ten milliter quantities were placed in sterile screw-cap tubes(15×150 mm) and heated in a water bath with controlled temperature(54.5±0.5° C.). The tubes were cooled in ice water and the bacteria wereenumerated on Tryptic Soy agar plus 7.5% NaCl, Violet Red Bile agar andTryptic Soy agar (all from Difco). Clostridium sporogenes (PA3679) GCI(50) DT spores prepared in beef heart infusion and stored in phosphatebuffer were added to the product to obtain an initial level of 10⁵spores per gram. Spores were enumerated on yeast extract agar containingsoluble starch, dextrose, sodium thioglycollate and sodium carbonatewith incubation of the plates in anaerobic BBL Gaspak jars (with H₂ +CO₂Gaspak generators) at 30° C. for 48 hours. Food products with bacteriaadded were tested in the screw-cap tubes (18×150 mm) or in thermal deathtime cans (208×006) designed by American Can Company both of which wereheated in miniature retorts. Diacetyl levels were determined by gaschromatography.

The data shown in Table 1 illustrates the enhancement of destruction bybacteria when using heat combined with commercial starter distillate ordiacetyl. A three log increase in S. aureus and a five log increase inS. Typhimurium and E. coli destruction was evident over that seen withheat alone.

                  TABLE 1                                                         ______________________________________                                        Influence of commercial starter distillate and diacetyl on                    enhancement of thermal inactivation of Staphylococcus aureus,                 Salmonella typhimurium and Escherichia coli, in pasteurized                   whole milk at 54° C. for 15* minutes.                                               Bacterial count on media** indicated                             Conc. of diacetyl                                                             (ppm)      pH      TSA       TSAS    VRB                                      ______________________________________                                        None (no heat)                                                                           6.65    4.4 × 10.sup.7                                                                    1.1 × 10.sup.7                                                                  1.5 × 10.sup.7                     None (heat only)                                                                         6.65    3.5 × 10.sup.6                                                                    1.5 × 10.sup.4                                                                  2 × 10.sup.5                       1,600 starter dist.                                                                      6.50    1.3 × 10.sup.3                                                                    2.5 × 10.sup.1                                                                  <10                                      800 starter dist.                                                                        6.60    2 × 10.sup.4                                                                      4 × 10.sup.1                                                                    <10                                      400 starter dist.                                                                        6.60    1.4 × 10.sup.5                                                                    5.5 × 10.sup.2                                                                  <10                                      1,600 diacetyl                                                                           6.60    2.7 × 10.sup.3                                                                    3 × 10.sup.1                                                                    <10                                      2,500 diacetyl                                                                           6.60    1.9 × 10.sup.3                                                                    8 × 10.sup.1                                                                    <10                                      ______________________________________                                         *Excluding a comeup time of 16 min to reach 54° C.                     **See footnote to TABLE 1 for media.                                     

Table 2 shows data relating to enhancement of thermal destruction ofClostridium sporogenes (PA3679) by starter distillate or diacetyl infoods heated at 110° C. There was a one log increase in destruction ofspores (after 15 and 30 minutes) in Sirloin Burger brand vegetable andbeef soup of Campbell Soup Co. and only slight to no enhancement inCream Style Corn.

                  TABLE 2                                                         ______________________________________                                        Influence of commercial starter distillate and diacetyl on                    enhancement of thermal destruction of Clostridium sporogenes                  (PA3679)* in various food substrates at 110° C.                        Food substrate     Survivors after heating** for                              and additives  pH      15 min   30 min                                        ______________________________________                                        Sirloin Burger 5.6     7.7 × 10.sup.3                                                                   3 × 10.sup.2                            Sirloin Burger +                                                              1600 ppm starter distillate                                                                  5.3     1.6 × 10.sup.2                                                                   2 × 10.sup.1                            Sirloin Burger +                                                              1600 ppm diacetyl                                                                            5.4     1.6 × 10.sup.2                                                                   2.5 × 10.sup.1                          Cream Style Corn                                                                             6.8     3.6 × 10.sup.3                                                                   3.3 × 10.sup.2                          Cream Style Corn +                                                            1600 ppm starter distillate                                                                  5.9     1.4 × 10.sup.3                                                                   1.8 × 10.sup.2                          Cream Style Corn +                                                            1600 ppm diacetyl                                                                            6.0     1.9 × 10.sup.3                                                                   1.1 × 10.sup.2                          ______________________________________                                         **includes a thermometric lag of 2 min.                                  

Table 3 shows data on inhibition of outgrowth of surviving spores inCream Style Corn and Sirloin Burger incubated at 37° C. No outgrowth wasseen in 4 of 4 cans even after 184 days in Cream Style Corn with starterdistillate as compared to all 4 of 4 cans showing outgrowth within threeweeks in the absence of starter distillate. That one of four cans showedoutgrowth in Cream Style Corn plus starter distillate in the presence ofadditives, indicates that surviving spores are injured and will requireadditional nutrients and anaerobic conditions to grow. The data alsoindicates that the inhibitory influence of starter distillate on sporessurviving in Sirloin Burger. Spores survived heat and were able to growrapidly in Sirloin Burger with additives (3 to 4 days). In the presenceof distillate there was a significant delay in outgrowth at 4 to 7 weekseven in the presence of additives. In the case of Sirloin Burger withdistillate and no additives there was no outgrowth of spores (none of 5cans) even after 124 days.

                  TABLE 3                                                         ______________________________________                                        Influence of commercial starter distillate on outgrowth of                    Clostridium sporogenes (PA3679) spores heated* in Cream Style                 Corn and Sirloin Burger (110° C. for 28 min) and incubated at          37° C.                                                                                                    No. of cans                                                                   positive for                                           pH                     outgrowth/                                             after   Length of incubation                                                                         total no. of                               Heating Substrate                                                                         heating (days)         cans                                       ______________________________________                                        Cream Style Corn                                                                          6.5     14-18          4/4                                        Cream Style Corn                                                              plus additives**                                                                          6.6     7-14           4/4                                        Cream Style Corn                                                              plus 1100 ppm                                                                 starter distillate                                                            plus additives                                                                            6.6     184            1/4                                        Cream Style Corn                                                              plus 1100 ppm                                                                 starter distillate                                                                        6.5     184            0/4                                        Sirloin Burger plus                                                           additives   6.0     3-4            5/5                                        Sirloin Burger plus                                                           additives and                                                                 1600 ppm starter                                                              distillate  6.0     7-60           5/5                                        Sirloin Burger plus                                                           1600 ppm starter                                                              distillate  5.5     124            0/5                                        ______________________________________                                         *In thermal death time cans; 1 × 10.sup.5 spores per container.         **Additives: Dextrose, sodium thioglycollate and sodium bicarbonate      

The data presented here indicates the enhancement of heat destruction ofStaphylococci and Salmonella and inhibition of surviving spores infoods. This low heat-starter distillate combination has a greatpotential for control of foodborne pathogens in low-acid foods. The useof commercial starter distillate combined with heat may serve as aneffective substitute for nitrites in canned, cured meats for providingbotulinum safety without the disadvantages of the nitrites. Contrary tothe prior art teachings, it has been found that diacetyl at the properlevels when combined with heat will provide effective control ofclostridium.

Further studies on the destruction and inhibition of Staphylococcusaureus in blood heart infusion media and Clostridium species (PA3679) incooked meat at a pH of 7.2 have found that Staphylococcus aureus can beinhibited at levels of 0.01% or greater diacetyl in BHI at 15, 37 and45° C. At higher levels of diacetyl 0.25, 0.5 and 1% and at highertemperatures the bacteria were rapidly killed. A five log increase indeaths of Staphylococcus aureus was due to the presence of 0.25%diacetyl at 55° C. It was also found that at 0.25% or higher levels,diacetyl is effective in killing Clostridium sporogenes (PA3679) incooked meat medium combined with temperatures of 37° C. or greater.

The diacetyl starter containing distillate is an ideal choice as apreservative since it is already an acceptable food flavoring additive.Diacetyl in the starter distillate is rather pungent and imparts odor tofood, however, the odor is limited by controlling the initialconcentration of diacetyl. The concentration of diacetyl combined withheating provides destruction of bacteria and only residual levels ofdiacetyl will remain. The residual level of diacetyl will continue tokill germinating bacterial spores. The low level of diacetyl thatremains is completely removed when the food is cooked prior to eating bythe consumer.

In considering the invention it must be remembered that the disclosureis illustrative only and that the scope of the invention is to bedetermined by the claims.

What is claimed is:
 1. The process for controlling foodborne pathogensin low-acid anaerobically packaged foods comprising the steps of:(a)adding from about 0.1% to about 1% diacetyl to a low-acid food product,and (b) heating the food product and diacetyl at a temperature ofgreater than about 75° and less than about 110° C. whereby the foodbornepathogens are substantially inhibited or killed.
 2. The process of claim1 wherein said diacetyl is in the form of a starter distillate.
 3. Theprocess of claim 1 wherein said foods are meat products.
 4. The processof claim 1 wherein said diacetyl is added in an amount of between about1100 and 2500 ppm and said food product is heated at a temperature ofgreater than about 75° and less than about 110° C. for between about 15and about 30 minutes.